1. Field of the Invention
The present invention relates to a programmable logic device (PLD). More particularly. it relates to a PLD including a cell block, hereinafter referred to as a macro cell, controlling a logic connection of an input/output of an input signal and an internally produced signal and a feedback thereof in accordance with contents stored in a programmable non volatile memory.
For example, in a PLD such as a programmable logic array PLA), a programmable array logic (PAL; registered trade mark) or the like. the above macro cell is provided to control a logic connection of the input/output of signals and the feedback thereof, using programmable non-volatile memory elements such as an electrically programmable read only memory (EPROM) and programmable switches controlling ON/OFF operations thereof and selecting each signal based on the contents stored in the programmable non-volatile memory elements, and thus realizes various logic constitutions.
2. Description of the Related Art
It is ideally desirable that the PLD can realize any logic circuit by a singe IC chip. Recently, the PLD products have been classified into several tens of types due to a variety of constitutions or structures. In view of this point, it is proposed that a singe chip (PLD) including several macro cells be constituted such that it can be replaced with a plurality of types of chips.
However, since a known conventional macro cell has an extremely simple constitution, it cannot realize a variety of control functions and, accordingly, it enables a replacement of only a few forms. On the other hand, to realize a LD including such a macro cell having an extremely simple constitution that can be replaced with a plurality of forms of chips, a logic designing in a programmable AND array and a programmable OR array must be carefully effected or devised. This leads to a restriction in the degree of freedom of the logic design in the entire PLD and a complication of the logic design per se, and thus is not preferable. An example of the known macro cell is disclosed in EPLD Handbook. 1985, p.20.about.p.28, published by ALTERA Corp., and an explanation thereof will be given later.